1. Field of the Invention
This invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns an improved methodology for starting up and operating a liquefied natural gas (LNG) facility employing a refluxed heavies removal column.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
Natural gas is frequently transported by pipeline from the supply source of supply to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in tile gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
In most LNG facilities it is necessary to remove heavy components (e.g., benzene, toluene, xylene, and/or cyclohexane) from the processed natural gas stream in order to prevent freezing of the heavy components in downstream heat exchangers. It is known that refluxed heavies columns can provide significantly more effective and efficient heavies removal than non-refluxed columns. However, one drawback of using a refluxed heavies removal column in conventional LNG facilities has been the significant delay in starting up the LNG facilities caused by the refluxed heavies removal column. The main reason for this delay in starting up the LNG facility was that during start-up, the reflux stream to the heavies removal column originated from a lower outlet of the heavies removal column. During start-up, the bulk of the feed stream entering the heavies removal column exited an upper outlet of the heavies removal column. As a result, only a small portion of the feed stream entering the heavies removal column during start-up exited the lower outlet and was available for routing back to the column as the reflux stream. As start-up progressed, the quantity of the feed stream available for use as reflux gradually increased to its optimum designed flow rate over a period of many hours or even days. However, the refluxed heavies removal column could not effectively remove heavies from the processed natural gas stream until the reflux stream was flowing at its designed rate. Thus, conventional start-up of an LNG facility employing a refluxed heavies removal column took many hours or even days.
A further disadvantage of conventional LNG plant start-up procedures was that the processed natural gas stream exiting the upper portion of the refluxed heavies removal column was simply flared because the elevated heavies concentration of this stream would freeze in downstream heat exchangers. Thus, because the bulk of the processed natural gas stream entering the refluxed heavies removal column during start-up exited the upper portion of the column and was subsequently flared, conventional start-up procedures for an LNG facility employing a refluxed heavies removal column wasted a significant portion of the processed natural gas stream.